1. Field of the Invention
The present invention relates to a rubber sheet joining apparatus and a method for overlapping and joining plural rubber sheet strips having different lengths, and for butt-joining side surfaces of the rubber sheet strips which are overlapped and joined. In particular, the present invention relates to a rubber sheet joining apparatus and a method for joining the rubber sheets to make a ply sheet for a body ply which is suitable for a low-profile tire.
2. Discussion of the Related Art
A body ply is used in a tire for a vehicle, wherein the body ply is reinforced with cords made of organic fiber or inorganic steel wire. In a general mass production, a large roll is made by winding a wide rubber sheet that is made in a manner that hundreds of the cords are aligned in parallel and a coat of rubber is applied onto the cords by a large calendar. The wide rubber sheet unwound from the large roll is cut to make a strip having a width corresponding to a width of the body ply determined according to the kind or the size of the tire by using a large cutting machine. A small roll having a width corresponding to a size of the tire is made by winding a ply sheet that is made by overlapping and joining end portions of the long strips cut off from the rubber sheet unwound from the large roll. The small roll is provided to the forming drum during tire forming.
Meanwhile, in a recent typical many kinds of small quantity production, a ribbon of rubber sheet reinforced with about ten to dozens of cords is cut to make a strips having a length corresponding to the width of the body ply, and a small roll is made by winding a ply sheet that is made by joining the strips at the side surfaces thereof. Then, the small roll is provided to the forming drum during tire forming.
In either case, a piercing treatment is performed through the ply sheet between the cords embedded therein to make through holes for extracting air between the ply sheets overlapped during manufacturing the tire. In either case of the calendar type and the ribbon type, since the piercing treatment is performed on the ply sheet reinforced with embedded cords while it runs continuously, the cords may be damaged. Also, there are problems that the through holes are clogged and air remains inside the body ply during tire forming and vulcanizing, since the operation of winding and rewinding is repeated due to a process for winding the ply sheet to make the small roll, and a process for rewinding the ply sheet from the small roll to wind the ply sheet around the forming drum to make the body ply.
Recently, a high rupture strength is required to the tire of a large size or the tire for SUV, and it is necessary for a low-profile tire to improve vehicle stability, wherein strict performance is required to the side portion of the low-profile tire. Therefore, such structure is used that the plural ply sheets are entirely overlapped and joined.
For example, there is a type disclosed in the Japanese Laid Open Patent Publication NO. 2002-240160, as a ribbon production type which is suitable to the many kinds of small quantity production, where a body ply has two ply layers made by entirely overlapped two ply sheets. However, in the Japanese Laid Open Patent Publication NO. 2002-240160, since the body ply is made of entirely overlapped two ply sheets having the same width, the width end portions of the two ply sheets coincide with each other, whereby a large repeated stress due to the tire running is concentrated on the width end portion of the body ply. Two ply sheets having different width are overlapped, so that the width ends of the two ply sheets do not coincide with each other to avoid the large repeated stress concentration on the width end portion of the body ply. In the mass production, two kinds of small rolls of ply sheets having different width are made by using a large calendar and a large cutting machine. The two ply sheets having different width are overlapped, so that the width ends of the two overlapped ply sheets do not coincide with each other. Thus, it can be prevented that the large repeated stress concentrates on the width end of the body ply.
As described above, the width ends of the two overlapped ply sheets are avoided to coincide with each other, so as to prevent the large repeated stress concentration on the width end of the body ply. However, it is difficult to make the width ends of the two overlapped ply sheets do not coincide with each other in the many kinds of small quantity production disclosed in the Japanese Laid Open Patent Publication NO. 2002-240160. On the other hand, in the mass production, the two small rolls having different width are provided around a forming machine to overlap the two ply sheets having different width. First, the ply sheet having the large width is wound on the forming drum from the one small roll having large width. Next, the ply sheet having the small width from the other small roll having the small width is wound on the forming drum over the ply sheet having the large width. Thus, two ply sheets having different width are overlapped and joined.
In the mass production, it is possible to manufacture the tire by using two kinds of ply sheets having different width. However, in this type, the two kinds of small rolls must be manufactured by using the large calendar and the large cutting machine, and the two kinds of small rolls need to be arranged around the forming machine. Thus, manufacturing facilities become large in size, and manufacturing cost increases. Further, there is a problem that a wide facilities space is required due to the large facilities. In order to meet a recent trend of tire of many kinds of small quantity, it is a problem to arrange the large calendar and the large cutting machine. Further, it is also difficult to gain inventory space or inventory control suitable for handling various small rolls for making tires of various kinds. Since the ply sheet unwound from the small roll has a joined portion where the end portions of strips are overlapped and joined circumferentially of the small roll, the thickness of the pry sheet on the circumference of the body ply is not uniform, and uniformity of the tire deteriorates.
In addition, if the cords are damaged during the piercing treatment for forming through hole to extract air, strength of the body ply in the tire deteriorates. Since operation of winding and unwinding is repeated after the piercing treatment, the through holes may be clogged. If the ply sheets are overlapped and joined without aligning through holes made through one ply sheets with through holes made through another ply sheet, it is difficult to extract air to make a problem that air remains after tire forming and vulcanizing processes.